1. Field of the Invention
The present invention relates to a method of wavelength locking and mode monitoring a tuneable laser. The invention also includes a method of compensating for laser degradation.
2. Description of the Related Art
Tuneable semiconductor lasers have a number of different sections through which current is injected, typically three or four such sections. The wavelength, power, and mode purity of the lasers can be controlled by adjusting the current in the various sections. Mode purity implies that the laser is at an operation point, i.e., in a combination of the three or four injected drive currents at which the laser is distanced from a combination of the drive currents where so-called mode jumps take place and where lasing is stable and side mode suppression is high.
Special wavelength controls are required with different applications. In the case of telecommunications applications, it is necessary that the laser retain its wavelength to a very high degree of accuracy and over very long periods of time, after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer and a typical time period is 20 years.
In order to be able to control the laser, it is necessary to map the behavior of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture.
It would also be very beneficial to lock the wavelength of a laser and ensure that the mode at which the laser operates can be controlled, so that the laser will operate as intended over a long period of time. By mode control is meant that when the laser is in use, either continuously or at regular intervals, the operation point of the laser can be optimized so as to exclude the risk of a mode jump to some other cavity mode. It would also be very beneficial to be able to automatically compensate operating lasers for degradation of the laser.
The present invention satisfies these desires.
The present invention thus relates to a method of locking the wavelength and monitoring the mode of a tuneable laser that includes two or more tuneable sections in which injected current can be varied, and of which sections at least one is a reflector section and one is a phase section. The laser will have been characterized with respect to suitable laser operation points which have been determined as different current combinations through different laser sections. The laser operates at a predetermined, selected operation point. The wavelength of the light emitted by the laser is detected with the aid of a wavelength- selective filter, and the laser is controlled in an iterative process in which alternated current through the reflector section of the laser, and at times the coupler section and the current through the phase section of said laser, is adjusted. The currents through the reflector section and the coupler section are adjusted to obtain a minimum in the relationship between power back and power forwards. The current through the phase section of the laser is adjusted so that the wavelength will be held constant, wherein the wavelength is measured against a wavelength reference.